As someone who doesn't plan on having children, I don't find the question rude in itself. It's how the person reacts to my initial "No" that make or breaks the situation. Some, especially new parents, seem to take it personal if someone their age isn't as excited over children as they are. It's extremely rare for the person to not follow up with a "why not?" and then continue the topic. "I don't want them" doesn't seem to be acceptable to many, especially if it comes from a fertile woman in her 30's...
Some either make the assumption that there's something wrong with you physically, or mentally if you don't have an interest in starting a family. Guess it's some sort of self-defence mechanic which is easier to apply than realizing that not everyone shares in your personal goals...